1. Field of the Invention
The present invention relates to landing gear mechanisms for semi-trailers and more particularly, to a mechanism for providing two speed ratios between the manually rotated drive shaft and the driven shaft which engages the raising and lowering mechanism for the landing gear.
2. Prior Art
It is common in landing gear for semi-trailers to provide a two speed selectable gear ratio between the drive shaft which is rotated by a manually operated crank and the driven shaft which operates the raising and lowering mechanism of the landing gear. This permits the landing gear to be quickly lowered to the ground and then more slowly extended in order to carefully lift the semi-trailer off of the tractor without damaging either. The provision of the two speed ratios is usually accomplished by having the drive shaft axially shiftable and supporting both a large diameter and a small diameter gear which can be shifted into engagement with corresponding small and large diameter gears on the driven shaft. This allows the driven shaft to be rotated at two substantially different speed ratios from the speed of rotation of the drive shaft.
An example of such mechanisms is disclosed for example in U.S. Pat. No. 3,892,141 which is assigned to the common assignee of the present invention. Since all of the gears are fixed to their respective shafts, it is necessary to provide a space between one of the pairs of gears on a shaft in order that the large gear on the other shaft will not be in engagement with its respective pinion gear on the first shaft when the other pair of gears are in engagement. This necessary spacing between the gears therefore requires a greater axial movement of the drive shaft when shifting from one gear ratio to another, and thus also requires a greater width of housing, than would be necessary if it were possible to have both pairs of gears resting immediately adjacent one another on their respective drive shafts.